1. Field of the Invention
The present invention relates to a current cutoff mechanism for a cell, and more particularly, to a current cutoff mechanism for a cell which may be used to ensure safety even when pressure inside a cell case rises excessively.
2. Description of the Related Art
For example, a rechargeable lithium-ion secondary cell, in which an organic solvent type electrolyte is sealed inside a cell case, has been widely used hitherto as a power source for portable devices such as portable telephones and personal computers.
If such a cell is overcharged or is charged using a higher current than a specified current at the time of recharging, however, trouble may occur in the cell. Gas is generated inside the cell case, and the generation of gas raises the pressure and temperature inside the cell. The cell case may undergoes swell, crack, etc., and the electrolyte contained therein may exude to the outside. These may adversely affect the device incorporating the cell.
Furthermore, if use of the cell is continued in the above-described abnormal condition, swelling of the cell progresses, which may pose a danger that the cell case will rupture. Therefore, it is necessary to immediately stop the use of the cell in abnormal conditions.
A description will be given of a conventional current cutoff mechanism for a cell, for example, a circular cell, for use in solving the above problems.
Referring to FIGS. 13 and 14, a cell lid 3 is fixed to a cell case 1 via a gasket 2 by caulking or other methods so as to hermetically seal the inside of the cell case 1, and an actuator 4 is disposed below the cell lid 3 so as to be displaced upward.
The cell lid 3 has vent holes 3a that allow the internal gas which has increased in pressure to escape therefrom to the outside when the actuator 4 is cleaved due to an increase in pressure inside the cell case 1.
The actuator 4 has an annular safety valve portion 4a formed by drawing or by other methods. The safety valve portion 4a includes a projection 4b projecting downward from the center, and radial grooves 4c formed on the lower surface around the projection 4b. The part of the actuator 4 where the grooves 4c are formed is made thin-walled, which allows the actuator 4 to be cleaved by an increase in pressure inside the cell case 1.
An insulating plate 5 is disposed under the actuator 4, through which the insulating plate 5, a hole 5a for passing the projection 4b of the safety valve portion 4a therethrough, and a vent hole 5b are formed.
Furthermore, a lead fixing member 6 is disposed under the insulating plate 5. The lead fixing member 6 has a hole 6a and a vent hole 6b respectively communicating with the hole 5a and the vent hole 5b of the insulating plate 5.
The projection 4b of the safety valve portion 4a is passed through the holes 5a and 6a of the insulating substrate 5 and the lead fixing member 6, and a lead 7 formed of a thin metal plate is attached to the leading end of the projection 4b. 
The thin-plate lead 7 has a joint portion 7a to be joined to the leading end of the projection 4b by welding or by other methods, whereby the actuator 4 and the lead 7 are electrically connected to each other. The other end of the lead 7 is connected to a power generating element 8 disposed in the lower part of the figure, so that a current path is formed between the power generating element 8 and the cell lid 3.
In a case in which the pressure inside the cell case 1 rises due to a problem inside the cell, gas which has increased in pressure flows from the vent holes 5b and 6b, as shown by arrow A in FIG. 14, and the force is applied to push the back of the safety valve portion 4a upward.
This force displaces the safety valve portion 4a upward, and concentrated stress acts on the joint portion 7a of the lead 7. By this concentrated stress, the joint portion 7a is broken, and the electrical connection between the lead 7 and the actuator 4 is also broken, thereby cutting off the current path in the cell.
This cuts off the flow of the current inside the cell, and blocks the increase in pressure inside the cell case 1. If the internal pressure of the cell further rises in spite of the cutoff of the current path, an even higher internal pressure is applied to the radial grooves 4c formed in the safety valve portion 4a. The thin-walled part of the actuator 4 having the grooves 4c is thereby cleaved, and the internal gas which has increased in pressure escapes from the cleaved part to the outside through the vent holes 3a. As a result, the cell can be prevented from being ruptured.
Portable devices, such as portable telephones, using such a conventional cell have become more compact, and there is a strong demand for size reduction of cells for use in the portable devices.
In the above-described conventional current cutoff mechanism, however, the actuator 4 must be large so that it operates without any trouble even when the internal pressure of the cell rises to a low pressure. For this reason, it is difficult to reduce the size of the conventional current cutoff mechanism.
Since the entire lead 7 is formed of a thin plate, when the internal pressure of the cell increases, the joint portion 7a is pulled and extended by the actuator 4 and is severed from the lead 7. Therefore, long fibrous burrs or the like are formed on a broken surface of the joint portion 7a and a broken surface of a broken hole 7b, which is formed by breaking the joint portion 7a. These burrs sometimes make it impossible to perfectly cut off the current path between the actuator 4 displaced up and the lead 7.
Accordingly, it is an object of the present invention to solve the above problems and to provide a high-performance current cutoff mechanism for a cell, which achieves size reduction and reliable current cutoff.
In order to achieve the above object, according to an aspect of the present invention, there is provided a current cutoff mechanism for a cell, including a cell case having a power generating element held therein; a cell lid for hermetically shielding the inside of the cell case; a diaphragm section protruding toward the inside of the cell case; an internal electrode connected to the power generating element and having a joint portion to be joined to approximately the center of the diaphragm section; and a fixing member for fixing the internal electrode inside the cell case while the internal electrode is joined to the diaphragm section, wherein a current path is formed between the cell lid and the power generating element via the diaphragm section and the internal electrode, a groove portion is formed to thin a portion in the vicinity of the joint portion of the internal electrode, and the internal electrode is broken from the groove portion to cut off the current path when the internal pressure of the cell case exceeds a predetermined value and the diaphragm section is displaced.
Preferably, the groove portion has a V-shaped cross section, and is annularly formed around the joint portion.
Preferably, a slit curved along the groove portion is formed in a part of the groove portion.
Preferably, two slits are formed opposed to each other across the joint portion.
Preferably, one of the slits is longer than the other slit.
The joint portion held between the two slits may be placed closer to one of the slits.
A break-inducing portion may be formed by slightly cutting a part of the groove portion in contact with both ends of the slit so that the internal electrode is broken from the side of the break-inducing portion when the internal pressure of the cell case rises.
According to another aspect of the present invention, there is provided a current cutoff mechanism for a cell, including a cell case having a power generating element held therein; a cell lid for hermetically shielding the inside of the cell case; a diaphragm section protruding toward the inside of the cell case; an internal electrode connected to the power generating element and having a joint portion to be joined to approximately the center of the diaphragm section; and a fixing member for fixing the internal electrode inside the cell case while the internal electrode is joined to the diaphragm section, wherein a current path is formed between the cell lid and the power generating element via the diaphragm section and the internal electrode, two arc-shaped slits are formed around the joint portion so as to be opposed to each other, the clearances between both ends of the two slits are made small, and the internal electrode is broken from the clearances when the internal pressure of the cell case exceeds a predetermined value and the diaphragm section is displaced.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.